gyp: Add dependency on snapshot.gyp:snapshot after 8bc6219.

Commit 8bc6219 ("[Tizen][Linux] Support getting thumbnail for devtools")
adds a call to ui::GrabViewSnapshotAsync but did not add a dependency on
the ui gyp target that contains the implementation of that function.

This broke the build of xwalk_browsertest on Linux when building with
component=shared_library:

  obj/xwalk/libxwalk_runtime.a(obj/xwalk/runtime/browser/devtools/xwalk_runtime.xwalk_devtools_delegate.o):xwalk_devtools_delegate.cc:function xwalk::XWalkDevToolsDelegate::GetPageThumbnailData(GURL const&):
  error: undefined reference to 'ui::GrabViewSnapshotAsync(aura::Window*, gfx::Rect const&, scoped_refptr<base::TaskRunner>, base::Callback<void (scoped_refptr<base::RefCountedBytes>)> const&)'

Fix it by adding the missing dependency.